1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine having a function of controlling an air/fuel ratio in each cylinder based on output of an air/fuel ratio sensor disposed in an exhaust air merging portion of the engine.
2. Description of Related Art
In most recent electronically controlled internal combustion engines, an air/fuel ratio sensor is disposed in an exhaust gas passage for detecting an air/fuel ratio of exhaust gas, and air/fuel ratio F/B control, whereby an air/fuel ratio (e.g., fuel injection quantity) in each cylinder is equally F/B (feedback) controlled such that an air/fuel ratio detected in the air/fuel ratio sensor accords with a target air/fuel ratio, is performed.
Furthermore, as described in JP2005-337194A, for example, the air/fuel ratio in each cylinder is estimated using a model, in which a detection value (air/fuel ratio at the exhaust air merging portion) of an air/fuel ratio sensor disposed in an exhaust air merging portion where exhaust gases from cylinders merge together is related with the air/fuel ratio in each cylinder. Based on the estimation result, each-cylinder air/fuel ratio control, whereby the air/fuel ratio (e.g., fuel injection quantity) in each cylinder is controlled so that a variation of the air/fuel ratios among the cylinders is small, is performed in order to improve air/fuel ratio control accuracy.
Also, as described, for example, in JP 2004-3513A corresponding to U.S. Pat. No. 5,672,817, in order to make an abnormal diagnosis of the air/fuel ratio sensor, an output change rate of the air/fuel ratio sensor in a predetermined period after the fuel injection cut-off in the engine is started is calculated as a responsivity detection value. Then, the output change rate of the air/fuel ratio sensor is compared with an abnormity determination value, to determine whether the air/fuel ratio sensor is abnormal (deterioration in responsivity).
Generally, although an abnormal electrical connection (e.g., a broken wire and a short circuit) in the air/fuel ratio sensor can be determined immediately after the engine is started (e.g., after an ignition switch is turned on), it cannot be determined, for example, whether the responsivity of the air/fuel ratio sensor is abnormal until the engine is in a predetermined operating condition (e.g., fuel injection cut-off state). In a system in which the air/fuel ratio F/B control, whereby the air/fuel ratio in each cylinder is equally controlled based on output of the air/fuel ratio sensor, is executed, the air/fuel ratio F/B control is started early on after the engine is started to reduce exhaust gas emission. Therefore, the air/fuel ratio F/B control is started at the time that a predetermined execution condition for the air/fuel ratio F/B control (e.g., the air/fuel ratio sensor is in an active state) is satisfied even before it is determined whether the responsivity of the air/fuel ratio sensor is abnormal. Then, if it is determined that the responsivity of the air/fuel ratio sensor is abnormal, the air/fuel ratio F/B control is forbidden at that point.
However, in the each-cylinder air/fuel ratio control, whereby the air/fuel ratio in each cylinder is controlled based on the output of the air/fuel ratio sensor, the air/fuel ratio in each cylinder is accurately estimated from the output of the air/fuel ratio sensor through the inverse operation, for example. Accordingly, the air/fuel ratio at the exhaust air merging portion varying with combustion in each cylinder needs to be detected in fast response in the air/fuel ratio sensor. As a result, higher-level responsivity of the air/fuel ratio sensor than general air/fuel ratio F/B control is required. When the each-cylinder air/fuel ratio control is started before it is determined whether the responsivity of the air/fuel ratio sensor is abnormal similar to the general air/fuel ratio F/B control, the each-cylinder air/fuel ratio control may be performed with the responsivity of the air/fuel ratio sensor deteriorated below the required level. In consequence, control accuracy in the each-cylinder air/fuel ratio control is deteriorated, and thereby the variation of the air/fuel ratios among the cylinders is large. Thus, a problem that exhaust gas emission is deteriorated is created.